Field of the invention. The present invention is directed to the field of holographic display systems and more particularly to the area of such systems utilizing broadband displays with dispersion control.
Description of the Prior Art. Systems commonly used in aircraft to display images of instruments in the field of view of a pilot are termed "heads-up displays" (HUD). HUDs generally employ conventional optical elements or holographic elements to project an image of an instrument into the pilot's field of view as the pilot looks through the forward window of the aircraft.
In U.S. Pat. No. 3,737,212 a first system of image projection is indicated in a HUD, wherein the image supplied by a cathode ray tube (CRT) is projected onto a "combining glass" in an aircraft cockpit. The image produced by the CRT is projected by optical elements including a mirror and a lens between the CRT and the combining glass. A second system described in that patent includes a combining glass, in the form of a spherical diffraction lens, which provides an image to the pilot as a consequence of a CRT image source having a concave parabolic face. While the second described system sets forth the desirability of providing a holographically recorded diffraction lens, it also emphasizes the necessity of using CRT's with rare earth doped phosphors that emit high energies of light in narrow wavelength bands, in order to reduce inherent aberrations.
In U.S. Pat. No. 4,218,111 a holographic HUD system is also disclosed. In that system, a complex optical system is used which includes relay lenses containing tilted and decentered optical elements employed to compensate for the aberration present in the holographic optical element used as a combiner.
These prior art patents illustrate conventional approaches to achieving a visibly perceivable HUD image against a bright background. However, the use of exotic phosphors to achieve bright narrow-band display illumination or expensive optics to provide image correction for broader-band sources have prevented widespread use of this otherwise desirable display concept.